1. Field of the Invention
The present invention relates to a wireless communications device, and more particularly, to an improved method of handling period PLMN searches in RRC connected mode for UMTS systems.
2. Description of the Prior Art
When a mobile station (MS) or user equipment (UE) is switched on, it attempts to make contact with a public land mobile network (PLMN). The particular PLMN to be contacted may be selected either automatically or manually.
The MS looks for a suitable cell of the chosen PLMN and chooses that cell to provide available services, and tunes to its control channel. This choosing is known as “camping on the cell”. The MS will then register its presence in the registration area of the chosen cell if necessary, by means of a location registration (LR), GPRS attach procedure, or IMSI (International Mobile Subscriber Identity) attach procedure.
If the MS loses coverage of a cell, or finds a more suitable cell, it reselects onto the most suitable cell of the selected PLMN and camps on that cell. If the new cell is in a different registration area, an LR request is performed.
If the MS loses coverage of a PLMN, either a new PLMN is selected automatically, or an indication of which PLMNs are available is given to the user, so that a manual selection can be made. Registration is not performed by MSs only capable of services that need no registration.
Please refer to FIG. 1. FIG. 1 is a diagram illustrating cells contained in different UTRAN registration areas (URAs). As an example, two URAs are illustrated U1, U2. URA U1 contains two cells C1, C2, and URA U2 also contains two cells C3, C4.
In GSM as in many other 2G systems, the radio resource protocol states were generally divided into two groups: the idle and the connected states. In the idle state no dedicated radio resources existed between the UE and the base station. In the idle state, however, the mobile station is far from being “idle.” There are several idle-mode tasks it must handle, tasks such as neighbor cell monitoring, cell reselection, paging channel reception, and broadcast data reception. In the connected state, however, a duplex radio connection is in place. The boundary between the idle and the connected mode is pretty clear; it is the existence of a dedicated radio resource. But in the new UTRAN system, this division is blurred. The idle state in UMTS is similar to GSM, as well as to those we find in other 2G systems: There is no uplink connection whatsoever. The UE has to monitor its radio environment regularly and, when necessary, perform a cell-reselection task. The reception of the broadcast system information and paging messages belong to the UE's idle-mode tasks. The connected state is different from the corresponding state in circuit switched 2G systems, but it has similarities with the packet-switched GPRS system. The connected mode is divided into four states: CELL_DCH, CELL_FACH, CELL_PCH, and URA_PCH. In the connected state there exists a logical RRC level connection between the UE and the UTRAN, but not necessarily a dedicated physical connection.
The CELL_PCH state is much like the idle mode because only the PICH is monitored regularly. The broadcast data (i.e., the system uplink activity is possible in the CELL_PCH state itself.
The URA_PCH state is quite similar to the CELL_PCH state, except that every cell change does not trigger a cell-update procedure. In this state an update procedure is only initiated if a UTRAN registration area changes, which is not done with every cell reselection. A state change to this state is requested by the UTRAN if it sees that the activity level of the UE is very low. The purpose of this state is to reduce the signaling activity because of cell updates. The drawback of this arrangement is that if the UTRAN wants to initiate data transmission while the RRC is in this state, it has to expand the paging area from one cell to several cells, possibly to the whole registration area because the location of the UE is not known with great accuracy.
The cell-reselection procedure, or as the 3GPP calls it, the cell reselection evaluation process, is performed in idle mode to keep the UE camped on a best cell. If the UE moves or the network conditions change, it may be necessary for the UE to change the cell it is camped on. This procedure checks that the UE is still camped on the best cell, or at least on a cell that is good enough for the UE's needs.
In normal idle mode, the UE has to monitor paging information and system information and perform cell measurements. The cell-reselection procedure will be triggered if the measurements indicate that a better cell has been found, or if the system information of the current cell indicates that new cell access restrictions are applied to the cell in question, such as cell barred. System information block 3 (SIB3) is an important message here because it tells the UE the quality parameter to measure, and also all the parameters for the cell-reselection evaluation algorithm. The neighbor cells to be measured are given in the neighbor cell list. In system information block 11 (SIB11), the results of these measurements are evaluated periodically. Based on these measurements the UE periodically evaluates the best cell status. If it seems that there is a better cell available, it will trigger a cell reselection procedure.
Please refer to FIG. 2. FIG. 2 is a diagram illustrating cells belonging to different PLMNs where no network sharing is used. As shown, cell C5 has a PLMN identity PLMN 1, and cell C6 has a different PLMN identity PLMN 2. Therefore, cells C5 and C6 belong to different telecommunications operators. Assuming a mobile station 10 is currently camped on cell C5, the mobile station 10 performs a periodic PLMN search for determining if there are nearby cells that belong to PLMNs with a higher priority. If the mobile station 10 determines that PLMN 2 has a higher priority than PLMN 1, the mobile station 10 will switch from cell C5 to cell C6 for utilizing PLMN 2 having a higher priority.
Suppose the mobile station 10 is in radio resource control (RRC) connected mode in the URA_PCH state and is camping on cell C5. When the mobile station 10 finds the cell C6 having a PLMN identity PLMN 2 that is higher than the PLMN identity PLMN 1 of the serving cell C5, the mobile station 10 will select cell C6 to camp on. When this happens, the mobile station 10 will enter idle mode from the URA_PCH state and then tries to establish a connection to register with PLMN 2 through cell C6.
If the mobile station 10 is in RRC connected mode in the CELL_PCH state, the same situation occurs if the mobile station 10 wishes to switch from cell C5 of PLMN 1 to cell C6 of PLMN 2 when PLMN 2 has a higher priority than PLMN 1. In this case, the mobile station 10 will enter idle mode from the CELL_PCH state and then tries to establish a connection to register with PLMN 2 through cell C6.
In the above examples illustrated in FIG. 2, the two cells C5 and C6 do not have a common PLMN identity. However, with network sharing becoming more popular since its introduction, new scenarios exist in which a cell can be shared among more than one operator, and therefore will have more than one PLMN identity. Please refer to FIG. 3. FIG. 3 is a diagram illustrating cells having common PLMN identities when network sharing is used. Due to network sharing, cell C7 has PLMN identities of PLMN 1 and PLMN 2 and cell C8 has PLMN identities of PLMN 2 and PLMN 3. Thus, both of the cells C7 and C8 have a common PLMN identity with PLMN 2.
According to the prior art, if the mobile station 10 is currently camped on cell C7 utilizing PLMN 1, the mobile station 10 performs a periodic PLMN search for determining if there are nearby cells that belong to PLMNs with a higher priority. If the mobile station 10 determines that PLMN 3 has a higher priority than PLMN 1, the mobile station 10 will switch from cell C7 to cell C8 for utilizing PLMN 3 having a higher priority.
Suppose the mobile station 10 is in RRC connected mode in the URA_PCH state and is camping on cell C7 utilizing PLMN 1. When the mobile station 10 finds the cell C8 having a PLMN identity PLMN 3 that is higher than the PLMN identity PLMN 1 of the serving cell C7, the mobile station 10 will select cell C8 to camp on. When this happens, the mobile station 10 will enter idle mode from the URA_PCH state and then tries to establish a connection to register with PLMN 3 through cell C8. Thus, even though the two cells C7 and C8 have a common PLMN identity PLMN 2, the mobile station 10 must still enter idle mode and establish a connection through the selected cell when switching from one cell to another since the mobile station 10 is switching from PLMN 1 to PLMN 3.
If the mobile station 10 is in RRC connected mode in the CELL_PCH state, the same situation occurs if the mobile station 10 wishes to switch from cell C7 of PLMN 1 to cell C8 of PLMN 3 when PLMN 3 has a higher priority than PLMN 1. In this case, the mobile station 10 will enter idle mode from the CELL_PCH state and then tries to establish a connection to register with PLMN 3 through cell C8.
Because the mobile station 10 moves to idle mode from either the CELL_PCH state or the URA_PCH state without signaling the UTRAN, the UTRAN has no idea that the mobile station 10 has entered idle mode. The UTRAN has to keep track of the radio resources for the mobile station 10 for a period of time, and if the UTRAN has no idea of the status of the mobile station 10, the radio resources are managed inefficiently if cells C7 and C8 have a common PLMN identity PLMN 2. In addition, the connection establishment time in idle mode is longer than the one in either the CELL_PCH state or the URA_PCH state.